Demand for cars that consume less fuel is strong these days. One way to improve the fuel efficiency is to reduce the overall weight of a car. In order to reduce the car weight, efforts are being made to reduce the weight of individual component parts of a car.
In this respect, it is considered highly important to reduce the weight of an oil pump, because 1) by reducing the weight of the pump and its peripheral parts and 2) by reducing the weight of frictional and rotary parts thereof, one can expect improved pumping capacity. For example, in case of a conventional oil pump for use in an automatic transmission, its part (pump case) is made of iron (mainly cast or diecast iron) and it weighs more than 5 kg. If the same part is made of an aluminum alloy, its weight will be less than 2 kg, which corresponds to about 60% weight reduction. Such a lightweight pump will show improved pumping capacity.
Heretofore, in generating a high-precision toothed part (gear) configured in a trochoid or involute curve by using a ferrous sintered part, the sizing technique has been employed in which pores remaining in the sintered part at the rate of 10-20% are partially closed by applying pressure, thus locally deforming the sintered part into the shape complementary to the metal mold, without giving any noticeable plastic deformation. The gear thus made has a high dimensional accuracy.
On the other hand, it is virtually impossible to apply such a sintering process to a part made of aluminum powder alloy, because, in case of an aluminum alloy powder, the oxide layer formed on the surface thereof tends to inhibit the diffusion and sintering. Sintering is applicable only in a eutectic liquid phase which appears at an extremely high temperature. But such a sintering operation tends to severely damage the microscopic and uniform metastable alloy phase obtained by the rapidly solidifying method or the mechanical alloying method and is thus practically meaningless. Furthermore, if a material made by solidifying aluminum powder should have pores at the rate of 10-20% as in the case of a ferrous sintered material, such a material could never be used for sliding members because its strength is extremely low.
Also, in generating a member made of aluminum powder alloy using the powdered metal technique, the aluminum alloy powder is molded and solidified in the cold and then is hot-forged. The heat produced by hot-forging tends to expand and shrink the mold and the solidified material, thus causing a change in the dimensions of the solidified material. It was therefore difficult to generate a part which is comparable in dimensional accuracy to a ferrous sintered part, with the heat-forging technique alone. If the solidified powder compact has a true density, what is done will be re-forging rather than sizing. Thus, it is impossible to improve the dimensional accuracy.
Also, if a rotor of an oil pump is made of one of various known aluminum alloys, such a rotor will have the following problems.
(1) If the rotor is made of an aluminum ingot metallurgy (I/M), which has heretofore been used as a slide member such as a piston or a bearing, such as AC8B and A390, its tooth surface would suffer severe wear damage resulting from pitching wear due to insufficient strength against frictional wear between aluminum alloys and surface pressure fatigue. Also, severe adhesion wear will appear at the end face and the outer peripheral portion due to seizure between the pump and the case. Further, when the rotor is rotating at high speed, fatigue failure may occur at the joint portion with the shaft due to insufficient strength of the rotor. Also, since cold forging cannot generate a precise and complicated shape, machining is further needed. As the percentage of Si increases, the primary crystal of Si becomes too coarse. This reduces the strength and toughness. On the other hand, in order to attain a sufficient high-strength temperature, the content of Fe has to be between 3 and 10%. But in case of ingot metallurgy, if the content of Fe is more than 5%, coarse needle-like structure will result, which lowers the toughness of the alloy. PA1 (2) If a rotor is made of a powder alloy composed of Al with Si contained at a high rate by use of the rapidly solidifying powder metallurgical technique, its thermal expansion coefficient becomes lower than that of the pump case material because it contains Si at a high rate. If it does a sliding movement at a temperature of about 150.degree. C., the clearance between the case and the rotor will increase, thus lowering the pumping capacity. Also, the alloy material of which the rotor is made is low in high-temperature strength. Thus, it is difficult to use this material for manufacturing a rotor which is used at a temperature of about 150.degree. C., i.e. a rotor to which the present invention relates. PA1 (3) If a rotor is made of a Al-Zn powder alloy composed of Al with Zn contained at a high rate, obtained by the rapidly solidifying powder metallurgical technique, its wear resistance will be poor though it shows good high-temperature strength due to remarkable age hardening characteristics. Thus, this material is not suitable as a material for a rotor for which a high wear resistance is required, that is, a rotor to which the present invention relates.
In order to retain excellent properties as a solidified body by use of high-performance aluminum alloy powder obtained by the rapidly solidifying method or the mechanical ironing method, the aluminum alloy powder particles have to be bonded together perfectly. But an oxide aluminum film covering each powder particle tends to inhibit such bonding. Generally, it is possible to remove sufficiently or break and destroy the oxide layer by selecting the heating and pressurizing conditions properly, so that the powder particles will be bonded together strongly, developing metallic bond and solid phase diffusion. The aluminum alloy part thus formed will show a sufficient strength.
An aluminum oxide layer is formed mainly while forming powder and heating the powder compact. In forming a part from an aluminum powder alloy, if the powder compact is heated to 300.degree. C. or higher, the crystal water adsorbed to the aluminum powder particles will evaporate and react with aluminum, thus forming a strong oxide layer on the powder particle surface. This will, as described above, inhibit the bond between the powder particles. The part thus made will have an insufficient strength.
Rapidly solidified aluminum powder containing transition elements such as Fe, Ni and Cr includes microscopic depositions of intermetallic compounds of these transition elements and aluminum (such as FeAl.sup.3, NiAl.sup.3 and CrAl.sup.3). The intermetallic compounds that deposit in the aluminum alloy powder have extremely small diffusion coefficients with respect to the aluminum matrix. Thus, when hot-forging such aluminum alloy powder, if the powder contains a transition element or elements in great amounts, the intermetallic compounds that have grown large when heated will inhibit the diffusion bond between the aluminum powder particles. This makes it difficult to provide an aluminum powder alloy member having sufficient strength and toughness.
The method of manufacturing an aluminum powder alloy member as described above is proposed e.g. in Japanese Patent Unexamined Publication 63-60265, in which the powder compact is subjected to heat treatment in the atmosphere in order to remove the water content which has been adsorbed to the surfaces of the powder particles. But as described above, the water content that has been removed will react with aluminum again, thus forming strong aluminum oxide layers on the powder particle surfaces. This inhibits the bond between particles. Further, in this publication, in order to sufficiently destroy the oxide layers on the powder particle surfaces and thus to bond the particles together, after heating the powder compact, a closed type hot-forging as a preparatory step is carried out and then hot-forging is carried out twice. Thus, this method tends to be costly.
Heretofore, trials have been made to improve the wear resistance of a sliding member made of aluminum alloy by adding Si crystals or hard particles such as SiC, TiC and Al.sub.2 O.sub.3 particles. But if the atmospheric temperature exceeds 100.degree. C. due to frictional heat during operation, the aluminum forming the matrix of the sliding member will begin to soften and the strength of the member lower. Thus, the frictional member becomes more prone to mechanical damage due to sliding and friction. Also, due to the shearing force acting while in frictional contact, Si crystals or hard particles may drop off, This reduces the wear resistance of the friction member.
An object of the present invention is, by use of the rapidly solidified powder metallurgical method together with the sizing method, to generate a rotor for an oil pump which is comparable in the dimensional accuracy and the wear resistance to a rotor made of a ferrous sintered material. Another object is to provide an economical manufacturing method in which the rate of the remaining pores in the solidified powder is adjusted to a level required for the sizing and thereby the drop in strength of the solidified powder is restricted, while keeping a microscopic and uniform metastable alloy phase which is necessary for higher wear resistance.